Generally, cardiac pacemakers have been used for many years to provide stimulation to a heart in order to regulate the pumping rate of the heart. The stimulation is in the form of electrical pulse signals. Cardiac pacemakers are available for providing a variable frequency of electrical pulse signals depending upon the requirements of the circulatory system. As used herein, the operating frequency of a cardiac pacemaker will be referred to as the "stimulation frequency".
Generally, the stimulation frequency of such cardiac pacemakers is relatively low when the body is at rest and increases in accordance with the stresses experienced by the body up to a predetermined maximum stimulation frequency predetermined for each individual case. Such variable frequency cardiac pacemakers utilize a sensing system for evaluating the degree of stress on the circulatory system to vary the stimulation frequency.
U.S. Pat. No. 4,228,803 discloses a variable stimulation frequency cardiac pacemaker in which the ventricle of the heart is stimulated. The stimulation electrode is located within the ventricle and also serves as a sensor. The cardiac pacemaker utilizes the well known fact that the time interval defined in an EKG between the Q peak and the T wave becomes shorter as body stress increases. In this cardiac pacemaker, the time interval between a stimulation electrical pulse and the following T wave is measured in a timing circuit and an evaluation circuit controls the frequency of the sequence of stimulation electrical pulses as a function of the measured time interval. This is referred to in the art as the "stimulus T principle".
The cardiac pacemaker disclosed in the '083 Patent can only be used in the ventricle and is not suitable at all for the atrium of the heart because there is no electrical signal corresponding to the T wave detectable in the atrium. This has the drawback that this cardiac pacemaker cannot be used with patients suffering from an excitation-producing or excitation-transmitting disease limited to the atrium. This is because no physiological synchronization will occur between the atrium and ventricle for such patients using this cardiac pacemaker.
In addition, it is possible that for such patients the stimulation of the ventricle could lead to a retrograde excitation of the atrium which usually results in a contraction of the atrium against the closed heart valve. This condition results in the physiological disadvantage of blood being pumped back into the veins rather than being pumped from the atrium into the main chamber of the heart.
U.S. Pat. No. 4,527,568 discloses a variable frequency cardiac pacemaker which utilizes the time interval between the stimulation electrical pulses and the T wave as a control value in a control circuit to control the stimulation frequency. This cardiac pacemaker combines an atrial-controlled ventricle pacemaker using the stimulus T principle. This cardiac pacemaker includes an additional sensor to provide an additional parameter related to the state of stress. The parameter can be pH value, the venous oxygen content, the body temperature, or the breathing velocity. This cardiac pacemaker can be used with patients suffering from an atrioventricular block (AV-block).
U.S. Pat. No. 4,108,148 discloses a cardiac pacemaker in which the ventricle is subjected to stimulation electrical pulses and the control of the operation is based on an electrical signal detected in the atrium of the heart. As used herein, the electrical signal in the atrium will be referred to as the "excitation signal". The operation of this cardiac pacemaker is related to the atrium-ventricle interchange time. As used herein, the atrium-ventricle interchange time is referred to as the "T.sub.AR ". The atrium-ventricle interchange time is often referred to as the "AR-interval".
The operation of the cardiac pacemaker disclosed in the '148 Patent assumes that the T.sub.AR is not constant with a functioning atrium-ventricle interchange and that it changes as a function of the stimulation frequency. In particular, it is assumed that T.sub.AR is relatively long for a relatively low stimulation frequency and relatively short for a relatively high stimulation frequency. The operation of the cardiac pacemaker adjusts the natural functioning of the heart by taking into account the shortening of the atrium-ventricle interval time when sensing a higher frequency of the autonomous atrial activity in response to ventricle stimulation.
U.S. Pat. No. 4,060,090 discloses a cardiac pacemaker in which the atrium-ventricle interval depends upon the stimulation frequency and becomes shorter as the stimulation frequency increases. This cardiac pacemaker utilizes this information for the stimulation of the ventricle.
The present invention provides a cardiac pacemaker which takes into account body stress by relatively simple and reliable measurements and is suitable for use with patients having excitation-producing or excitation-transmitting disease of the atrium.